1. Field of the Invention
The present invention relates generally to an optical device, and more particularly to an optical filter used in coarse wavelength division multiplexing (CWDM).
2. Description of the Related Art
In fast development of computer and network, people may obtain data through internet in an easy and fast way. The photoelectric communication provides a fast and mass data transmission therefore optoelectronic industry becomes more and more important. In optoelectronic field, the fast improvement is the combination of electronics and optics, and optical networking is the one we want to develop first.
The optical networking is a technique using optical fiber to be the communicating media of data transmission to transmit analog or digital data through laser ray between different systems, such as computer system and telephone system. The laser ray has a frequency higher than the electric wave, and the lost of laser ray in the optical fiber is very little that it has higher transmission speed and efficiency than any conventional communication system.
Coarse wavelength division multiplexing (CWDM) is a very common technique in optical networking which has a filter film and a multiplexer to integrate various channels with different frequencies in single optical fiber, and separate the different wavelength into plural optical fibers at receive end by a demultiplexer. The filter film of CWDM has Fabry-Perot resonators, each of which includes plural high reflective layers and low reflective layers stacked together. The following is a sample of the sequence of the high and low reflective layers of the filter film of CWDM in sequence from an air side (Air) to a substrate side (Ns):
Air/H(LH)^2 2L(HL)^3 H(LH)^3 4L(HL)^3 H(LH)^4 6L(HL)^4 H(LH)^3 6L(HL)^3 H(LH)^4 6L(HL)^4 H(LH)^3 6L(HL)^3 H(LH)^4 6L(HL)^4 H(LH)^3 4L(HL)^3 H(LH)^3 2L(HL)^2 H/Ns
wherein: H indicates a high reflective layer with a basic optical thickness (λ0/4); L indicates a low reflective layer with a basic optical thickness (λ0/4); and λ0 indicates central wavelength of incident rays.
With the filter as described above and the multiplexer it may integrate plural channels with different frequencies in single optical fiber.
However, as shown in FIG. 1, the filter as described above cannot reduce the stopbands of the channels. Each channel has a constant bandwidth, and the neighboring channels will interfere with each other when they have great stopbands that the communication quality of the optical networking will be decreased.